Power supplies generally include circuitry necessary for allowing the supply to slowly ramp up to its full voltage potential in order to reduce the inrush current demand on the power supply. Power supply circuitry is also provided for monitoring the supply voltage and for turning off the supply in event of a fault, such as overvoltage or excessive current draw. In the event of a fault, circuitry must also be provided for restarting the power supply after the fault has cleared.
Prior art softstart and fault reset circuitry is based on a fault signal discharging a capacitor and monitoring of the capacitor voltage. When the capacitor voltage has decreased below a predetermined threshold, the power supply voltage is reapplied. This design works well only when the fault signal is long enough to insure that the capacitor will fully discharge. In the case of large capacitors and short fault signals, however, the capacitor will not fully discharge and the power supply will restart from a voltage level above the desired restart voltage level.
Subsequent prior attempts at providing softstart circuitry include the addition of a latch, such as shown in FIG. 1, to control the capacitor discharge path and compel the user of such a device to externally reset the fault latch before restarting the power supply. Although functional, such a manual reset puts the timing burden onto the user.
An additional enhancement of prior art power supply softstart and reset circuitry is illustrated in FIG. 2 wherein a full NPN differential comparator is utilized to sense capacitor voltage and clear the fault latch automatically when the capacitor has discharged below a predetermined threshold. Although this circuit provides for automatic fault latch reset without user intervention, the circuit requires a large number of active and inactive circuit elements, increasing the size and cost of the circuit, which accordingly reduces circuit manufacturing yield and reliability.